8. Suppose that a student performed the experiment and calculations perfectly ex

Question

8. Suppose that a student performed the experiment and calculations perfectly except for the following. Unknown to the student, Cu(NO3)2.3H20 instead of CuSO4·5H2O was used to make up the known Cu+2 solution. Using his data the student found a percentage of copper in the brass samples of 50.0%, would this percentage be correct? If so, why? If not, what is the correct percentage? Show how you got your answer. 9. If a solution of 0.750 g of CuSO4·5H20 in 80.0 mL of solution has an absor- bance of 0.100, what is the absorbance of a solution of 1.200 g of CuSO4 5H20 in 50.0 mL of solution? Show your set-up and the result of your calculation

Explanation / Answer

8)

Molar mass of Cu(NO3)2.3H2O = 241.602g

Mass% of Cu = (63.55 /241.602)×100= 26.30

Molar mass of CuSO4.5H2O = 249.677g

Mass % of Cu =( 63.55/249.677)×100 = 25.45

Difference in mass % of Cu = 26.30-25.45 = 0.85%

So, Cu mass % error= (0.85 /26.30 )×100 = 3.23

So, there is +3.23% error

So,the actual % of Cu in brass = 50 - ((3.23/100)×50) = 48.39

9)

Beer's law: A = cl

where A is absorbance, is absorption coefficient, c is concentration and l is path length

Since and l are the same for both measurements:

A2/A1 = c2/c1

A1 = 0.100

c1 = 0.750/80.0 = 0.009375 g/mL

c2 = 1.200/50.0 = 0.0240 g/mL

A2/0.100 = 0.0240/0.009375

Absorbance A2 = 0.256

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